There is a number stamped on top of the piston. This is the fractional part of it's size, as in 74.XXXmm. There is a number stenciled on the side of the cylinder sleeve where it hangs out the bottom of the block. This is the fractional part of the bore size, as in 75.XXXmm. Subtract piston size from bore size and you should get something in the .050-.055mm range. Now, this is the minimum, best clearance and was this way when the parts were new. Obviously, things will be different now that the parts are used.
Measuring the bores yourself is pretty hard but a close visual inspection can give you a good idea of their condition. You want to see very little, if any, vertical scraping and nothing you can actually feel. Measuring the pistons is easier. Mic the skirts about 1/2" up from the bottom, front to rear. Subtract that from the stenciled bore size for a general idea of wear. Granted, clearance will probably be a bit more than your math indicates because of bore wear. However, the bores generally wear less than the pistons because they're harder.
Mine was pretty close to the wear limit but I decided to just do a hone w/ new rings. I'm going into my 5th season now on it and it runs fine. Compression is very good, right up around 150. I will probably tear it down again sometime in the next couple years and this time, it will get bored. All in all, I'm quite happy with the service I've gotten out of just a ring replacement.
